Now look at the receptive field on the right. Clearly more of the inhibitory surround lies in either the same gray as the center or in darker areas. Only a small portion lies in a brighter area than the excitatory center.
Consequently, one would expect that the excitatory center would have a larger response on the diagonal than for a receptive field not on a diagonal. Hence there appears a brightness enhancement on the diagonals of this diagram. You, of course, noticed that there is no brightness on the diagonals in this figure. I show why in the next diagram.
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